Failed NOx sensor module 'A' or its internal electronics; Damaged, chafed, or corroded wiring between the NOx sensor and the ECM; Loose or contaminated electrical connector at the NOx sensor or ECM; Blown fuse or poor ground feeding the NOx sensor heater circuit; CAN bus communication fault affecting the sensor's data line; Software or calibration issue in the ECM requiring a reflash; Aftermarket or incorrect NOx sensor installed that does not communicate properly

DTC codes / U02xx · Lost communication with a control module / U029D

U029DLost Communication With NOx Sensor A

Q: What does code U029D mean?

Lost Communication With NOx Sensor A. U029D is a generic network communication code indicating that the engine control module (ECM) has lost the controller area network (CAN) signal from the NOx sensor module labeled 'A'. The ECM monitors a dedicated serial data line or CAN bus for messages from that sensor and sets this code when expected periodic messages stop arriving for a calibrated time.

U029D is a generic network communication code indicating that the engine control module (ECM) has lost the controller area network (CAN) signal from the NOx sensor module labeled 'A'. The ECM monitors a dedicated serial data line or CAN bus for messages from that sensor and sets this code when expected periodic messages stop arriving for a calibrated time.

SystemNetwork

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaLost communication with a control module

Symptoms you may notice

Illuminated check engine light, often with no other noticeable symptoms
Reduced engine power or limp mode if the ECM cannot verify NOx levels
Increased diesel exhaust fluid (DEF) consumption as the system defaults to a protection strategy
Rough running or hesitation if the ECM substitutes a default NOx value and alters injection timing
A message on the dashboard indicating emissions system service required
Possible failure to complete an emissions inspection because the monitor is not ready

Common causes, most likely first

Failed NOx sensor module 'A' or its internal electronics
Damaged, chafed, or corroded wiring between the NOx sensor and the ECM
Loose or contaminated electrical connector at the NOx sensor or ECM
Blown fuse or poor ground feeding the NOx sensor heater circuit
CAN bus communication fault affecting the sensor's data line
Software or calibration issue in the ECM requiring a reflash
Aftermarket or incorrect NOx sensor installed that does not communicate properly

How to diagnose and fix it

Connect a scan tool and verify U029D is current, then check for other communication codes that might point to a shared bus fault
Inspect the NOx sensor connector and wiring for corrosion, chafing, or loose pins, and repair as needed
Check the fuse and ground circuits that supply power to the NOx sensor module
Measure CAN bus voltage and resistance at the sensor connector to confirm the data line is intact
If wiring and power are good, substitute a known-good NOx sensor 'A' and clear the code to see if communication returns
Perform an ECM software update if the manufacturer has published a calibration for this communication fault
After repair, clear codes and complete a drive cycle to verify the monitor runs and the code does not reset

Why a good NOx sensor still sets U029D

A common oversight is replacing the NOx sensor when the real fault is a chafed wire harness near the exhaust or transmission, so always perform a wiggle test on the harness before condemning the sensor.
This code can be set by a weak battery or charging system because low voltage during cranking interrupts the sensor's boot-up handshake with the ECM, leaving a communication fault stored.
Some vehicles require a specific relearn or adaptation procedure after NOx sensor replacement; skipping it can cause U029D to return even with a new sensor.
A faulty aftertreatment control module or a second NOx sensor on the same bus can disrupt communication for sensor 'A', so check for other U-codes before assuming the sensor itself failed.
U029D does not always mean the sensor is dead; it can be a temporary communication dropout caused by water intrusion into a connector that dries out and allows the code to clear on its own.

Can I keep driving with U029D?

You can usually drive the vehicle short distances to a repair shop, but avoid heavy loads and long trips because the ECM may enter a reduced-power strategy and emissions compliance is compromised. If the engine enters limp mode, stop driving and arrange a tow.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.